1. Field of the Invention
The present invention relates to ink jet printing apparatuses and ink jet printing method, in which ink is ejected through nozzles provided in a print head to print an image.
2. Description of the Related Art
In ink jet printing apparatuses, even a small amount of nozzle-by-nozzle structural variation that may occur during the process of manufacturing their print heads can affect the ejecting amount and direction of ink through each nozzle, which causes deterioration in image quality such as streaks and uneven density in printed images. In order to eliminate such uneven image density, there has been proposed a multipath printing method (separate printing technique) in which a print head scans a predetermined area on a printing medium multiple times to print images sequentially. In the multipath printing method, the print head passes multiple times each printing area and the predetermined area are printed through different groups of nozzles.
The multipath printing method has the advantages of reducing uneven image density due to eject amount distribution among nozzles and/or misaligned ink landing. At the same time, the multipath printing technique has a lower throughput as the number of scanning for printing before the completion of an image increases.
Hence, a bi-directional printing method has been adopted to increase for throughput degradation, in which printing is performed not only during forward scanning of a print head but also during backward scanning of the print head in the reverse direction after the printing operation during the forward scanning.
However, bi-directional multipath printing with a relatively small number of scanning (two to six times) may suffer from so-called “time difference unevenness” and therefore image deterioration. Specifically, time difference unevenness is a phenomenon particularly prominent when reversing a high-speed print head (kickback) and printing large-sized (approximately A4-sized or larger) images, and it is therefore necessary to eliminate time difference unevenness to increase throughput when printing large-sized images.
Here will be described “time difference unevenness” with reference to FIGS. 21 to 23.
The time difference unevenness will be explains to an example the case two-scan printing with reference to FIG. 21. In bi-directional two-scan printing, a head carriage starts a first scanning from the left-most printing start position shown in FIG. 21 and about half the group of nozzles on the upstream end side of conveying direction of the nozzle arrays are used to print nearly printing half dots which should be printed. Subsequently, the scanning direction of the carriage is reversed at the right-hand edge and the printing medium is conveyed by a predetermined feed amount (half of the nozzle arrays). Then during a second scanning, to-be-printed dots other than printed on the printing area through the first scan are printed in a printing area A on the downstream end side, while nearly printing half dots are printed in a printing area B on the upstream end side, as is the case in the first scanning. Subsequently during a third scanning, the scanning direction of the carriage is further reversed at the printing start position side and the printing medium is conveyed by a predetermined feed amount. Then approximately half of all the to-be-printed dots are printed through the upstream end side of the nozzle arrays, as is the case in the first scanning, while to-be-printed dots other than previously printed in the printing area B are printed through the downstream end side. This image will be completed by repeating the sequence above.
In the case above, focusing on the printing area A in the image printing start region, the time interval between the first scan printing and the second scan printing at the left-most printing start position side is the sum of the time for printing by the time for two printing scan and the time for reversing of the head carriage. Additionally, if the time for paper feeding that is performed simultaneously with the reversing of the head carriage is longer than the reversing time, the time difference is also added. Meanwhile, in the adjacent printing area B, the time for reversing (reversing of the head carriage and paper feeding) is only required, and therefore, the time interval between the first scan printing and the second scan printing is to be very short.
Such a time difference increases with a greater image printing width of main scanning direction. As for large-sized printing of A4-size or larger paper, for example, the difference at the edge between the inter-scan time differences in the printing areas A and B increases.
In the case above, the image density (color tone) is accordingly different between the adjacent printing areas at the edges of the image, it may cause deterioration in image quality as uneven density at the predetermined width of printing area (a paper feed pitch).
Four-path printing for printed by four-scan will also be described with reference to FIGS. 22A and 22B, and three-path printing for printed by three-scan will be described with reference to FIG. 23.
In FIG. 22A shows the aspect that an image is printed by four scans printing. FIG. 22B shows the image of the left edge part of each printing area, i.e. the image of the edge where first scan is started. In FIG. 22A, during a first scanning for printing 14-001, approximately one-fourth of the to-be-printed dots are printed in a printing area A, and then during a second scanning for printing 14-002, another one-fourth of the to-be-printed dots are printed. In the left edge part of the printing area A, it require the time for back-and-forth scanning and reversing of the carriage between the time to be printed by the first printing scanning and to start to print by the second printing scanning. Subsequently, it require the time only for reversing of the carriage between the time to be printed by the second printing scanning and to start to print by the third printing scanning 14-003. It require the time for back-and-forth scanning and reversing of the carriage between the time to be printed by the third printing scanning and to start to print by the forth printing scanning 14-004. Meanwhile, On the left part area of the printing area 14-B, it require the time only for reversing of the carriage between the time to be printed by the first printing scanning 14-002 and to start to print by the second printing scanning 14-003. It require the time for back-and-forth scanning and reversing of the carriage between the time to be printed by the second printing scanning 14-003 and to start to print by the third printing scanning 14-004. It require the time only for reversing of the carriage between the time to be printed by the third printing scanning 14-004 and to start to print by the forth printing scanning 14-005.
On the left edge part of the image shown in FIG. 22B, such a difference in the combination of time differences is one of the factors that cause uneven image density (color tone). And on the right edge part of the image, same uneven image density occurs, and then comb-like strip-shaped unevenness due to alternate occurrence of uneven image density both edge part. Even in the case of three-path printing such as shown in FIG. 23, time difference unevenness will occur similarly at the edges of the printing areas A and B.
In order to reduce such time difference unevenness, there has been known a method in which a printing ratio are set to their respective different values of printing ratio per each group of nozzles (refer to Japanese Patent Laid-Open No. 2002-292910, for example).
Meanwhile, time difference unevenness is an image deterioration caused by a time difference between a previous scanning and printing and the next scanning and printing, and the degree of occurrence of unevenness depends significantly on the type of printing medium. That is, the degree of ink penetration depends on the type of printing medium, which results in a significant difference in the degree of unevenness. Therefore, the most suitable control method for reduction of time difference unevenness and, more particularly, the most suitable printing ratio set for each printing scan is different for each type of printing medium. For this reason, the most suitable printing ratio for reduction of time difference unevenness has conventionally been set for each specific type of printing medium assumed to be used in an ink jet printing apparatus.
However, the most suitable printing ratio for reduction of time difference unevenness varies according to the type of printing medium. Therefore, in the printing apparatus that the most suitable printing ratio for reduction of time difference unevenness is arranged only for a specific type of printing medium, it may be impossible to reduce image deterioration due to time difference unevenness sufficiently when unspecified types of printing media are used.